Posterization or "banding" is an undesirable series
of stepped colour gradients in what should be a smooth transition
of colour. It will most frequently be encountered in backgrounds
to images that should have nice smooth out-of focus colours. Also
it will especially occur when images are being heavily compressed
from a large file to a small size for web viewing.

This tutorial offers advice on how to prevent posterization from
ruining your images and also gives some ideas to reduce it once
it has crept in.

Posterization may be summarised as "Insufficient colour
data spread too far apart". Think in terms of your Photoshop
levels histogram representing the tones in an image. If there
are not many tones present and you adjust the levels to open up
the histogram you will see gaps appearing giving a comb effect.
The gaps are missing data. Other technical articles on the web
will explain this thoroughly, but if like me, you are more interested
in knowing how to prevent posterization or fix it once you have
it ....read on.

As I shoot in 16 bit RAW rather than 8bit jpeg , I rarely encounter
posterization but a recent photoshoot of a very cute little owl
with a smooth blurred background gave me all sorts of problems
to edit and get my pictures onto the web without looking awful.
Here is the little chap proudly holding a mouse before flying
back to feed it to its young.

Take a careful look at the background and you will see some nasty
colour transitions. Below is a 100% crop of part of the image
and you can clearly see the rainbow coloured steps.

I first noticed the posterization in the image in the Adobe RAW
converter while I was viewing at normal screen size and I knew
it might be troublesome. However, when I viewed at 100% or full
size the problem went away. So, first lesson is that the image
from the camera was not really posterized but I was seeing a Photoshop
artifact through not viewing at actual size where one pixel in
the image is represented by one pixel on the screen.
However, had I have shot this image in jpeg or under-exposed it
and had to open up the levels I might not have been so lucky.

If you look at the owl in the image you will notice that as the
late afternoon sun was coming from the left, the opposite side
of its body is in shade. I therefore boosted the brightness of
the dark side by moving the shadows slider in the RAW converter
to the right. Also I added some saturation to bring out the colour
intensity a bit. Finally I added some noise reduction before opening
it in Photoshop. Once in Photoshop I further improved the contrast
issues with the shadow/highlights tool and sharpened the image.
Unfortunately these are all steps that can aggravate posterization
in an image ! When I came to reduce the size of the full sized
image to a tiny web-sized jpeg the posterization was very prominent
and totally unacceptable.

What can be done to minimise posterization?

The single most important thing that can be done to prevent
posterization is to shoot in RAW. This is a 16 bit format whereas
jpeg is 8 bit. This means that there is twice the data to work
with. 8 Bit is not always sufficient to allow the smooth transitions.
Once processed in 16bit, save in a lossless format such as Tiff.

When editing in Photoshop, use adjustment layers where
possible and generally keep editing to a minimum. The most problematical
editing tools which will aggravate posterization are any that
effectively open up the levels, thus exposing gaps in the data
- so significant brightening through levels, curves or shadows/highlights
should all be used with care.

Do not under-expose your images as you will be forced to brighten
them later in post processing. This will open the levels and will
invite posterization. You will also minimise noise into the bargain
by "shooting to the right."

Noise reduction also enhances posterization so only use the minimum
required and go easy on those sliders in post processing ! I set
the colour and luminance detail sliders to zero and used just
a whiff of Luminance and colour noise reduction.

Go easy with Hue/saturation tools and do not add sharpening to
the problem areas. This will usually mean painting on sharpening
using layer masks.

You can make a selection of the problem area of the image and
leave it alone, this usually means the background. Then select
inverse (which will usually select your subject) and process as
required. This is what I did with my little owl image. I brightened
the dark side of the owl with shadows/highlights and increased
the hue/saturation without affecting the background. I then flattened
the image layers again and saved as a Tiff file. You can also
use a layer mask if you are familiar with this process.

I also had to resize the image and save to web using as little
compression as I could get away with, without making the filesize
too large for web viewing.

Here is my second attempt at processing the image - much better
I think you will agree.

How can Posterization be removed ?

There is an old saying that "prevention is better
than cure." This is very appropriate in the case of posterization.
It iscertainly easier to prevent
it in the first place than to try to remove it later as all you
can really do is disguise it.

My first tip would be to go back to the original image and reprocess
it again using the suggestions in the previous paragraph - as
I did with my little owl image above.

So what can be done to remove posterization
? Surprisingly, the best way of dealing with it in Photoshop
is to disguise it by adding noise to the parts of the image showing
the effect.This means first making a selection
of the area or using a layer mask and then applying noise as described
below. It is important that the noise should
be added after all editing has been completed and the image has
been resized its final size but before the final sharpening has
been applied.To add the noise,select Filter/Noise/Add Noise and the Add Noise dialog
box appears:

Amount determines how much noise is added. Only a small amount
needs to be added but this will depend on the size of your image.
I usually add more than is necessary, then go to Layers and back
off the percentage opacity from 100% to what looks best.Too little
and the posterization is still visible, too much and noise becomes
intrusive.

The Distribution control should be set to Gaussian. Finally, the
Monochromatic box should be checked. Checking this box makes sure
that the noise that is added is tonal rather than colour noise.
Click OK.

Finally, as an alternative, you can download a Photoshop plug-in
to repair posterized histograms (Google it). You will probably
have to pay for these plugins and I do not know how affective
they are as I have never had to resort to using them.